The invention relates to a low-pressure gas discharge lamp, in particular a mercury low-pressure gas discharge lamp, comprising a gas discharge vessel with a gas filling containing mercury, and comprising a phosphor coating, electrodes and means for igniting and maintaining a gas discharge.
Light generation in mercury low-pressure gas discharge lamps is based on the principle that during the gas discharge a plasma develops in the mercury-containing gas filling, which plasma does not only emit a percentage of visible light but also a very high percentage of short-wave UV radiation. The phosphors in the phosphor coating of the lamp absorb this UV radiation and re-emit it as visible light.
In this process, the phosphors are the last member of the energy transfer chain in which electric energy is converted in the lamp into visible light. The efficiency of a low-pressure gas discharge lamp with a phosphor layer thus decisively depends on the electro-optical efficiency of the phosphor, i.e. the extent to which the generated UV light is completely absorbed in the phosphor and the extent to which the generated visible light subsequently leaves the lamp in the direction of the observer.
A drawback of known mercury low-pressure gas discharge lamps resides in that the action of short-wave UV radiation on the phosphor coating as well as the recombination of mercury ions and electrons on the phosphor surface or the incidence of excited mercury atoms and electrons on the phosphor layer cause the emissivity of the phosphors, particularly of BaMgAl10O17:Eu2+, to decrease in the course of time. This manifests itself as a reduction of the electro-optical efficiency in the course of the service life.
This degradation is particularly substantial under the influence of VUV radiation having a wavelength below 200 nm, and it does not only manifest itself as a deterioration of the electro-optical efficiency but also by a shift of the color point.
DE 197 37 920 describes that degradation of the phosphors in a low-pressure gas discharge lamp can be reduced in that the discharge vessel forms an inner bulb that is transparent to electromagnetic radiation, which inner bulb is surrounded by an outer bulb, and the inside of the outer bulb and/or the outside of the inner bulb is provided with a phosphor layer.
In this manner it is precluded that recombination of mercury ions and electrons on the phosphor surface, or the incidence of excited mercury atoms and electrons on the phosphor layer causes the emissive power of the phosphor to be reduced in the course of time. Degradation of the phosphors due to the action of short-wave UV radiation, however, is not influenced thereby.
Therefore, it is an object of the invention to provide a low-pressure gas discharge lamp with a phosphor coating, in which degradation of the phosphors during the service life of the lamp is reduced.
In accordance with the invention, this object is achieved by a low-pressure gas discharge lamp comprising a gas discharge vessel with a gas filling containing mercury, and comprising electrodes, means for igniting and maintaining a gas discharge, and a phosphor coating containing at least one phosphor layer and a UV-C phosphor and a phosphor that can be excited by UV-C radiation.
The UV-C phosphor in the phosphor coating acts like a converter. It absorbs the portion of mercury radiation in the high-energy VUV range with an emission maximum at approximately 185 nm and emits radiation in the longer UV-C wavelength range of 230 to 280 nm. As a result, degradation of phosphors by the high-energy VUV radiation is precluded.
Within the scope of the invention it is preferred that the UV-C phosphor comprises an activator selected among the group consisting of Pb2+, Bi3+ and Pr3+ in a host lattice.
These UV-C radiation-emitting phosphors combine a very good absorption in the VUV range with a high absorption coefficient xcex5=10,000 l/cmxc2x7mol to 100,000 l/cmxc2x7mol and an emission quantum yield  greater than 90%. Unlike other phosphors, they are hardly degraded by the VUV radiation.
It may be preferred that the UV-C phosphor comprises Pr3+ and La3+. These phosphors emit UV-C radiation comprising two bands in the wavelength range between 220 and 265 nm. The absorption maximum of halophosphate phosphors and three-band phosphor mixtures is found in said wavelength range.
It may also be preferred that the UV-C phosphor comprises Pr3+ and Y3+.
It is particularly preferred that the UV-C phosphor is selected among the group consisting of LaPO4:Pr, LaBO3:Pr, LaB3O6:Pr, YBO3:Pr, YPO4:Pr and Y2SiO5:Pr.
Particularly advantageous effects of the invention in relation to the state of the art are obtained when the UV-C phosphor is selected among the group consisting of YPO4:Bi and LuPO4:Bi. These UV-C phosphors are transparent to visible radiation and Hg emission at 254 nm.
Within the scope of the invention it is preferred that the phosphor that can be excited by UV-C radiation contains an activator selected among the group consisting of Ce(III), Tb(III), Eu(II), Mn(II) and Sb(III). These phosphors with the oxidizable activator ions Ce(III), Tb(III), Mn(II), Eu(II) and Sb(III) are particularly affected by a degradation through photo-oxidation and profit to a particular extent from the invention.
In accordance with an embodiment of the invention, the UV-C phosphor is present in a first phosphor layer and the phosphor that can be excited by UV-C radiation is present in a second phosphor layer.
In accordance with a preferred embodiment of the invention, the UV-C phosphor has a grain size of 10 nm less than d less than 500 nm and is present in a first phosphor layer, and the phosphor that can be excited by UV-C radiation is present in a second phosphor layer.
A phosphor layer containing a UV-C phosphor having a grain size in the nano-range forms a very dense layer that satisfactorily shields the phosphor that can be excited by UV-C radiation from the VUV radiation originating from the mercury plasma. In addition, this very dense layer causes the recombination of mercury ions and electrons on the surface of the phosphor layer to be reduced.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.